On the differences between horizontal-to-vertical spectral ratios caused by earthquakes and ambient noise—A case study of vertical-array observations in Northern China

Abstract Horizontal-to-vertical spectral ratio (HVSR) of both earthquake and ambient noise are applied to perform inversions of the subsurface velocity structure of a certain site, however, their mechanisms differ significantly. A comparative study on earthquake HVSR (EHV) and noise HVSR (NHV) for the same site is rare, hence, herein, a case study is conducted on the Xiangtang vertical-array observation station, located in the city of Tangshan, northern China. The earthquake and noise records observed at this array station are used to investigate the spectral characteristics of EHV and NHV. Differences between the EHV and NHV are compared, and the possible causes for the differences are explored. Further, the mechanisms of the EHV and NHV are discussed, and the suggestions about HVSR inversion are offered. The results of this study show that the EHV and NHV significantly differ in the spectral shape and amplitude in the medium-to-high frequency band. Typically, the amplitude of the observed EHV in the frequency band larger than two times of the predominant frequency (f0) is higher than that of the observed NHV; the mechanisms of the EHV and NHV show clear differences. For frequencies larger than 2f0, the observed EHV is better explained by the theoretical body-wave HVSR, and the observed NHV is better explained by the theoretical full-wave HVSR. Therefore, when performing an inversion to obtain subsurface velocity structure using HVSR, one must carefully distinguish the data sources and choose the correct forward method to obtain more reliable results.